1. Field of the Invention
The present invention relates to localization of a position during surgery. The present invention relates more specifically to a system that facilitates combined electromagnetic and optical localization of a position during stereotactic surgery, such as brain surgery and spinal surgery.
2. Description of Related Art
Precise localization of a position has always been important to stereotactic surgery. In addition, minimizing invasiveness of surgery is important to reduce health risks for a patient. Stereotactic surgery minimizes invasiveness of surgical procedures by allowing a device to be guided through tissue that has been localized by preoperative scanning techniques, such as for example, MR, CT, ultrasound, fluoro and PET. Recent developments in stereotactic surgery have increased localization precision and helped minimize invasiveness of surgery.
Stereotactic surgery is now commonly used in surgery of the brain. Such methods typically involve acquiring image data by placing fiducial markers on the patient's head, scanning the patient's head, attaching a headring to the patient's head, and determining the spatial relation of the image data to the headring by, for example, registration of the fiducial markers. Registration of the fiducial markers relates the information in the scanned image data for the patient's brain to the brain itself, and utilizes one-to-one mapping between the fiducial markers as identified in the image data and the fiducial markers that remain on the patient's head after scanning and throughout surgery. This is referred to as registering image space to patient space. Often, the image space must also be registered to another image space. Registration is accomplished through knowledge of the coordinate vectors of at least three non-collinear points in the image space and the patient space.
Currently, registration for image guided surgery is completed by a few different methods. First, point-to-point registration is accomplished by the user to identify points in image space and then touch the same points in patient space. Second, surface registration involves the user's generation of a surface (e.g., the patient's forehead) in patient space by either selecting multiple points or scanning, and then accepting or rejecting the best fit to that surface in image space, as chosen by the processor. Third, repeat fixation devices entail the user repeatedly removing and replacing a device in known relation to the fiducial markers. Such registration methods have additional steps during the procedure, and therefore increase the complexity of the system and increase opportunities for introduction of human error.
Through the image data, quantitative coordinates of targets within the patient's body can be specified relative to the fiducial markers. Once a guide probe or other instrument has been registered to the fiducial markers on the patient's body, the instrument can be navigated through the patient's body using image data.
It is also known to display large, three-dimensional data sets of image data in an operating room or in the direct field of view of a surgical microscope. Accordingly, a graphical representation of instrument navigation through the patient's body is displayed on a computer screen based on reconstructed images of scanned image data.
Although scanners provide valuable information for stereotactic surgery, improved accuracy in defining the position of the target with respect to an accessible reference location can be desirable. Inaccuracies in defining the target position create inaccuracies in placing a therapeutic probe. One method for attempting to limit inaccuracies in defining the target position involves fixing the patient's head to the scanner to preserve the reference. Such fixation may be uncomfortable for the patient and creates other inconveniences, particularly if surgical procedures are involved. Consequently, a need exists for a system utilizing a scanner to accurately locate positions of targets, which allows the patient to be removed from the scanner.
Stereotactic surgery utilizing a three-dimensional digitizer allows a patient to be removed from the scanner while still maintaining a high degree of accuracy for locating the position of targets. The three-dimensional digitizer is used as a localizer to determine the intra-procedural relative positions of the target. Three-dimensional digitizers may employ optical, acoustic, electromagnetic or other three-dimensional navigation technology for navigation through the patient space.
Different navigational systems have different advantages and disadvantages. For example, electromagnetic navigation systems do not require line-of-sight between the tracking system components. Thus, electromagnetic navigation is beneficial for laproscopic and percutaneous procedures where the part of the instrument tracked cannot be kept in the line-of sight of the other navigation system components. Since electromagnetic navigation allows a tracking element to be placed at the tip of an instrument, electromagnetic navigation allows the use of non-rigid instruments such as flexible endoscopes. However, use of certain materials in procedures employing electromagnetic tracking is disadvantageous since certain materials could affect the electromagnetic fields used for navigation and therefore affect system accuracy.
Comparatively, optical navigation systems have a larger working volume than electromagnetic navigation systems, and can be used with instruments having any material composition. However, the nature of optical navigation systems does not accommodate tracking system components on any portion of an instrument to be inserted into the patient's body. For percutaneous and laproscopic procedures, optical navigation systems typically track portions of the system components that are in the system's line of sight, and then determine the position of any non-visible portions of those components based on system parameters. For example, an optical navigation system can track the handle of a surgical instrument but not the inserted tip of the surgical instrument, thus the navigation system must track the instrument handle and use predetermined measurements of the device to determine where the tip of the instrument is relative to the handle. This technique cannot be used for flexible instruments since the relation between the handle and the tip varies.
Stereotactic surgery techniques are also utilized for spinal surgery, in order to increase accuracy of the surgery and minimize invasiveness. Accuracy is particularly difficult in spinal surgery and must be accommodated in registration and localization techniques utilized in the surgery. Prior to spinal surgery, the vertebra are scanned to determine their alignment and positioning. During imaging, scans are taken at intervals through the vertebra to create a three-dimensional pre-procedural data set for the vertebra. However, after scanning the patient must be moved to the operating table, causing repositioning of the vertebra. In addition, the respective positions of the vertebra may shift once the patient has been immobilized on the operating table because, unlike the brain, the spine is not held relatively still by a skull-like enveloping structure. Even normal patient respiration may cause relative movement of the vertebra.
Computer processes discriminate the image data retrieved by scanning the spine so that the body vertebra remain in memory. Once the vertebra are each defined as a single rigid body, the vertebra can be repositioned with software algorithms that define a displaced image data set. Each rigid body element has at least three fiducial markers that are visible on the pre-procedural images and accurately detectable during the procedure. It is preferable to select reference points on the spinous process that are routinely exposed during such surgery.
See also, for example, U.S. Pat. No. 5,871,445, WO 96/11624, U.S. Pat. No. 5,592,939 and U.S. Pat. No. 5,697,377, the disclosures of which are incorporated herein by reference.